2020, Opherden, D., Nizar, N., Richardson, K., Monroe, J.C., Turnbull, M.M., Polson, M., Vela, S., Blackmore, W.J.A., Goddard, P.A., Singleton, J., Choi, E.S., Xia, F., Williams, R.C., Lancaster, T., Pratt, F.L., Blundell, S.J., Skourski, Y., Uhlarz, M., Ponomaryov, A.N., Zvyagin, S.A., Wosnitza, J., Baenitz, M., Heinmaa, I., Stern, R., Kühne, H., Landee, C.P.

We report on a comprehensive characterization of the newly synthesized Cu2+-based molecular magnet [Cu(pz)2(2-HOpy)2](PF6)2 (CuPOF), where pz=C4H4N2 and 2-HOpy=C5H4NHO. From a comparison of theoretical modeling to results of bulk magnetometry, specific heat, μ+SR, ESR, and NMR spectroscopy, this material is determined as an excellent realization of the two dimensional square-lattice S=12 antiferromagnetic Heisenberg model with a moderate intraplane nearest-neighbor exchange coupling of J/kB=6.80(5) K, and an extremely small interlayer interaction of about 1 mK. At zero field, the bulk magnetometry reveals a temperature-driven crossover of spin correlations from isotropic to XY type, caused by the presence of a weak intrinsic easy-plane anisotropy. A transition to long-range order, driven by the low-temperature XY anisotropy under the influence of the interlayer coupling, occurs at TN=1.38(2) K, as revealed by μ+SR. In applied magnetic fields, our H1-NMR data reveal a strong increase of the magnetic anisotropy, manifested by a pronounced enhancement of the transition temperature to commensurate long-range order at TN=2.8 K and 7 T. © 2020 authors.

2019, Rößler, S., Koz, C., Wang, Z., Skourski, Y., Doerr, M., Kasinathan, D., Rosner, H., Schmidt, M., Schwarz, U., Rößler, U.K., Wirth, S.

A detailed experimental investigation of Fe1+yTe (y = 0.11, 0.12) using pulsed magnetic fields up to 60 T confirms remarkable magnetic shape-memory (MSM) effects. These effects result from magnetoelastic transformation processes in the low-temperature antiferromagnetic state of these materials. The observation of modulated and finely twinned microstructure at the nanoscale through scanning tunneling microscopy establishes a behavior similar to that of thermoelastic martensite. We identified the observed, elegant hierarchical twinning pattern of monoclinic crystallographic domains as an ideal realization of crossing twin bands. The antiferromagnetism of the monoclinic ground state allows for a magnetic-field–induced reorientation of these twin variants by the motion of one type of twin boundaries. At sufficiently high magnetic fields, we observed a second isothermal transformation process with large hysteresis for different directions of applied field. This gives rise to a second MSM effect caused by a phase transition back to the field-polarized tetragonal lattice state.